1. Field of the Invention
This invention relates to a demodulator of a sound-multiplexed TV signal or a multi-channel stereo signal, and more particularly to a demodulator including demodulation means of stereo signal and Second Audio Program (hereinafter referred to as SAP) signal.
2. Description of the Related Art Including Information Disclosed .sctn..sctn. 1.97-1.99
A sound-multiplexed TV signal is transmitted in the form of a mixed signal of five signals components, which are a main channel signal (50 Hz.about.15 kHz) of L+R, a stereo pilot signal of 1.multidot.f.sub.H (horizontal scanning frequency; 15.734 kHz), a sub-channel signal of L-R, an SAP signal and a telemetery signal. The main channel signal is a signal of L+R signal. The sub-channel signal is an AM-DSB signal of L-R signal or L-R signal which has been subjected to noise reduction process such as a dbx noise reduction process and modulated with 2.multidot.f.sub.H signal. The SAP signal is an FM signal modulated with 5.multidot.f.sub.H signal. The telemetery signal is an FM signal modulated with 6.5.multidot.f.sub.H but is not demodulated to listen TV sound. All of the above five signals are FM-modulated to be broadcasted.
In a TV set, the received signal is FM-demodulated to reproduce the five signals. First, the L-R signal is reproduced from the sub-channel signal by using 2.multidot.f.sub.H signal which is generated by a PLL (Phase Locked Loop) circuit. The PLL circuit includes a first voltage controlled oscillator (hereinafter a voltage controlled oscillator is referred to as a VCO), a frequency divider dividing the output frequency of the first VCO and a phase comparator comparing the divided frequency and the stereo pilot signal of 1.multidot.f.sub.H to control the output frequency of the first VCO by the comparison output. The demodulated L-R signal is applied to a matrix circuit with the L+R signal of the main channel signal to separate L and R signals. If the reproduced L-R signal was subjected to the noise reduction process, the L-R signal is decoded to produce an original L-R signal prior to the application to the matrix circuit. Additionally, the SAP signal is FM-demodulated by a PLL FM-demodulator which includes a second VCO having a free-running frequency of about 5.multidot.f.sub.H and a phase comparator comparing the reproduced SAP signal and the output from the second VCO to control the output frequency of the second VCO.
Thus, a TV set has two VCO's to demodulate the sound-multiplexed signal. The two VCO's are independently operated. Therefore, no problem occurs when the SAP signal is demodulated. However, an interference between the output signals from the two VCO's occurs when the SAP signal is not received. This is because it is very hard that the free running frequency is precisely adjusted to the 5.multidot.f.sub.H. If the SAP signal is not received, the second VCO oscillates at the free running frequency. A deviation of the free running frequency from the 5.multidot.f.sub.H generates a cross modulation with the output from the first VCO.